The winding operation consists substantially of transferring the yarn from a starting package and winding it on a rigid tube in order to form a structure wound in the form of cross turns and known as a bobbin, and during said transfer clearing the yarn of its imperfections and defects such as lumps, groups, naps, weak points, flocks etc. Said defects are eliminated by cutting out the defective portion and joining the ends.
This joint can be made either by a proper knot such as a fishermans knot or a weavers knot produced by a mechanical knotter, or by a pneumatic or friction joint in which the fibres of the cut ends are untwisted, intermixed and then retwisted to thus restore continuity to the cut yarn without introducing the hardly relevant irregularity represented by an actual knot.
The removal of yarn defects is commonly known as yarn clearing in that the defect is detected by a yarn clearer which is sensitive to yarn defects and can either itself break the continuity of the yarn or operate a separate cutting member.
Any discontinuity in the yarn causes the bobbin to undergo braking so that it stops, the yarn ends are picked up by mobile suckers and moved to the joining devices or knotters, the joined yarn is returned to its normal position and winding is recommenced, the bobbin and its drive roller being driven up from rest to the operating speed, which is generally of 600-1600 m/minute.
The winding speed is determined--within the limits of the possible winding machine performance--by the quality and count of the yarn to be wound.
The overall productivity of the operation is determined by the winding speed, the time taken by the overall intervention cycle and the actual number of interventions to be made.
It is therefore apparent that if a certain yarn is wound at a too high speed, the increased productivity resulting from the increase in speed is compromised by the down times deriving from the increase in the number of interventions required to restore the yarn continuity due to the greater number of yarn breakages. The bobbin is normally driven by a rotating roller--of right cylindrical or slightly tapering conical shape--which is kept in contact along a generator common to the two members. The technical problem to which the present invention relates derives from the fact that during the winding operation the rotating roller does not change its shape or size, whereas the bobbin continuously changes its size due to the increasing amount of yarn wound on it.
If the drive takes place under perfect friction, the peripheral speed of the drive roller is substantially equal to the linear winding speed of the yarn.
The yarn is guided so that it winds on the bobbin in a spiral arrangement using a yarn guide of various shapes or spiral grooves formed in the surface of the driving roller, in which the yarn engages.
By the action of such devices, the yarn is distributed over the bobbin surface by means of periodical travel along the bobbin generator.
The closer together the turns, the more dense is the bobbin and vice versa.
As the size of the bobbin increases, the linear yarn winding speed is kept substantially constant--this being a necessary condition for proper outcome of the operation--but the angular speed of the bobbin decreases linearly.
As the yarn travels along the contact generator in constant time, the number of turns wound for each travel stroke of the yarn guide reduces slightly but continuously for each wound layer.
As the bobbin forms it acquires an ever increasing inertia because of the increase in mass and its progressive distancing from the axis of rotation.
The first stage in the intervention cycle which commences with the cutting or tearing of the yarn by the passage of a defective portion through the yarn clearer is the braking of the bobbin so that its speed decreases to zero.
The brake must therefore absorb the kinetic energy possessed by the rotating bobbin, and its stoppage time is substantially proportional to said kinetic energy.
Generally, the bobbin is braked by a mechanical shoe brake--or equivalent type--operated by pressurised fluid such as compressed air, which is distributed by a solenoid valve which operates following the yarn discontinuity signal.
The drive roller is provided with its own braking devices, such as an inverter acting on its drive motor. To prevent damage to the bobbin it is desirable that the two braking actions take place independently, by withdrawing the bobbin and roller away from each other when the yarn discontinuity signal occurs at the commencement of the intervention cycle.
The operations subsequent to the stoppage can take place only when the bobbin is at rest.
In the known art the intervention cycle is effected as shown in the scheme of FIG. 1.
The duration of the intervention cycle is fixed and is divided into a fixed time available for stoppage and a fixed time for executing the other operations to be carried out during the intervention. After the stoppage time has passed, the bobbin must be completely at rest because otherwise the other intervention operations cannot be properly carried out, for instance it would be impossible to grip the end of the yarn on the bobbin side if this is still rotating.
The drive and control unit for the members which sequentially carry out the various operations of the intervention cycle is a mechanical system--such as a shaft provided with a series of cams so that when rotated, said cams sequentially encounter the drives for the various members, which consequently operate in sequence--or an equivalent electrical control system.
In this arrangement, the various intervention operations are performed sequentially by various members operated in accordance with a program of operation initiation times which are rigid and cannot be changed.
To be more precise, it should be noted that certain preliminary operations, such as moving the suckers into the correct position for seeking and picking up the yarn ends, these suckers being in their rest position at the commencement of the intervention cycle, can commence while the bobbin is still moving, but the actual operations of the intervention cycle subsequent to braking can only commence when the bobbin is properly at rest.
If the bobbins to be produced are small or if the operating speed is low, the time taken by those preliminary operations which can be carried out while the bobbin is still moving is longer than the bobbin stoppage time, and there are therefore no problems.
The fixed time allowed for bobbin stoppage must therefore correspond to the time required for absorbing the maximum kinetic energy which the bobbin can possess, and thus to its maximum possible winding speed, its maximum possible size and its maximum possible density. This time must then be increased by a certain safety margin to take account of any reduction in the efficiency of the braking system.
The current tendency in bobbin production is to increase winding speed and to maintain it when producing large-diameter bobbins. It is apparent that the criterion of assigning a fixed available time for bobbin stoppage based on the maximum kinetic energy which it can assume leads in most cases to a considerable time wastage because this fixed assigned time is necessary only when the bobbin has reached its maximum scheduled size and rotates at the maximum speed scheduled for this size.
This is very important because this time wastage--even if only of the order of a few seconds--is repeated during every intervention cycle for restoring yarn continuity, and this cycle can take place hundreds of times.
The deriving technical problem which the present invention solves is to assign a bobbin stoppage time within the intervention cycle which is no longer fixed but is variable, and corresponds substantially to the time which the braking device would require at any given moment to bring the bobbin to rest, this time depending on the kinetic energy of the bobbin at the moment of this operation.